1. Field of the Invention
The present invention relates to an organic electroluminescent element.
2. Description of the Related Art
Organic electroluminescent elements using an organic material are expected to be promising in applications such as inexpensive, full-color, wide flat-panel display devices of the solid luminescent type and writing light source arrays, and thus development of the elements has been carried out extensively. An organic luminescent element is generally composed of a light emitting layer, and a pair of opposing electrodes between which the light emitting layer is disposed. When an electric field is applied between the two electrodes, electrons are injected from the cathode and holes are injected from the anode. Luminescence (light emission) is a phenomenon in which energy is emitted in the form of light, when the electrons and the holes are recombined in the light emitting layer and the electrons return to the highest occupied molecular orbital from the lowest unoccupied molecular orbital.
Therefore, organic electroluminescent elements have an advantage in that they are used for display devices and illumination and can make such display devices lighter in weight and thinner in thickness. Organic electroluminescent elements have excellent characteristics as described above, however, generally, in an organic EL device using an organic electroluminescent element, a refractive index of each layer constituting the organic EL device is higher than a refractive index of air. For example, in an organic EL device using an organic electroluminescent element, a refractive index of an organic thin-film layer such as a light emitting layer is 1.6 to 2.1. Therefore, emitted light is totally reflected at an interface between layers with ease, and the light extraction efficiency is less than 20%, resulting in loss of most of the light.
For example, in a generally known organic EL device, an organic EL display unit includes a pair of electrode layers and an organic compound layer disposed between the pair of electrodes, on a substrate. The organic compound layer includes a light emitting layer, and the organic EL device emits light, which has been emitted from the light emitting layer, from a light extracting surface side thereof. In this case, a total reflection component, which is light having a critical angle, cannot be extracted at an interface between the organic compound layer and the light extracting surface or each electrode layer, and thus a problem arises that the light extraction efficiency is low.
For the reasons described above, there have been various organic EL devices proposed to improve the light extraction efficiency, including organic EL device, in which a light extraction member (e.g., lens) adapted to control an optical path of light emitted from a light emitting layer to cause the emitted light emerge outside the light extracting side is provided on the optical path; and organic EL devices in which the light extraction efficiency in the forward direction is improved by a fine resonator (microcavity) structure one of a pair of electrode layers sandwiching the light emitting layer is provided as a reflective electrode, and the other electrode is provided as a semi-transmissive/semi-reflective electrode.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-207656 proposes an organic EL display device which includes an organic EL element having at least a light emitting layer between electrodes, and a transparent substrate disposed via a micro-lens layer on one surface side of the organic EL element, wherein the micro-lens layer includes a plurality of convex-shaped lens elements which are arrayed so as to form a plane, and a flattened layer which is formed so as to cover the convex-shaped lens elements, and wherein the refractive index of the convex-shaped lens elements is higher than that of the flattened layer.
In addition, there has been also known a method of improving the light extraction efficiency in a forward direction by a fine resonator (microcavity) structure in which one of a pair of electrodes sandwiching an organic compound layer is provided as a reflective electrode, and light emitted from a light emitting layer is caused to resonate by the other electrode (see Japanese Patent (JP-B) No. 4174989).
Lenses for use in these organic EL devices generally have a high refractive index equivalent to a refractive index of a light emitting layer for the purpose of improving the light extraction efficiency. However, a new problem arises that when such a lens having a high-refractive index is attached to a microcavity element, there is a significant difference in chromaticity between the case where a display device utilizing the organic EL device is viewed from the front side and the case where the display device is viewed in an oblique direction, and when the display device is viewed from an oblique direction, light having a component inferior in chromaticity is unfavorably contained.